Search

Two isoforms of inositide-dependent phospholipase C β1 (PI-PLCβ1) are generated by alternative splicing (PLCβ1a and PLCβ1b). Both isoforms are present within the nucleus, but in contrast to PLCβ1a, the vast majority of PLCβ1b is nuclear. In mouse erythroid leukemia cells, PI-PLCβ1 is involved in the regulation of cell division and the balance between cell proliferation and differentiation. It has been demonstrated that nuclear localization is crucial for the enzymatic function of PI-PLCβ1, although the mechanism by which this nuclear import occurs has never been fully characterized. The aim of this study was to characterize both the mechanism of nuclear localization and the molecular function of nuclear PI-PLCβ1 by identifying its interactome in Friend's erythroleukemia isolated nuclei, utilizing a procedure that coupled immuno-affinity purification with tandem mass spectrometry analysis. Using this procedure, 160 proteins were demonstrated to be in association with PI-PLCβ1b, some of which have been previously characterized, such as the splicing factor SRp20 (Srsf3) and Lamin B (Lmnb1). Co-immunoprecipitation analysis of selected proteins confirmed the data obtained via mass spectrometry. Of particular interest was the identification of the nuclear import proteins Kpna2, Kpna4, Kpnb1, Ran, and Rangap1, as well as factors involved in hematological malignancies and several anti-apoptotic proteins. These data give new insight into possible mechanisms of nuclear trafficking and functioning of this critical signaling molecule.

A proteomic-based search for novel substrates of Akt was undertaken in C2C12murine muscle cells. Our data demonstrate that Akt isoform 2 phosphorylates Ankrd2 at Serine 99 in response to H2O2stimuli, regulating muscle differentiation rate.

Activation of Akt-mediated signaling pathways is crucial for survival, differentiation, and regeneration of muscle cells. A proteomic-based search for novel substrates of Akt was therefore undertaken in C2C12murine muscle cells exploiting protein characterization databases in combination with an anti–phospho-Akt substrate antibody. A Scansite database search predicted Ankrd2 (Ankyrin repeat domain protein 2, also known as ARPP) as a novel substrate of Akt. In vitro and in vivo studies confirmed that Akt phosphorylates Ankrd2 at Ser-99. Moreover, by kinase assay with recombinant Akt1 and Akt2, as well as by single-isoform silencing, we demonstrated that Ankrd2 is a specific substrate of Akt2. Ankrd2 is typically found in skeletal muscle cells, where it mediates the transcriptional response to stress conditions. In an attempt to investigate the physiological implications of Ankrd2 phosphorylation by Akt2, we found that oxidative stress induced by H2O2triggers this phosphorylation. Moreover, the forced expression of a phosphorylation-defective mutant form of Ankrd2 in C2C12myoblasts promoted a faster differentiation program, implicating Akt-dependent phosphorylation at Ser-99 in the negative regulation of myogenesis in response to stress conditions.

Recent data indicate that some PKC isoforms are translocated to the nucleus, in response to certain stimuli, where they play an important role in nuclear signaling events. To identify novel interacting proteins of conventional PKC (cPKC) at the nuclear level during myogenesis and to find new PKC isozyme-specific phosphosubstrates, we performed a proteomics analysis of immunoprecipitated nuclear samples from mouse myoblast C2C12 cells following insulin administration. Using a phospho(Ser)-PKC substrate antibody, specific interacting proteins were identified by LC-MS/MS spectrometry. A total of 16 proteins with the exact and complete motif recognized by the phospho-cPKC substrate antibody were identified; among these, particular interest was given to eukaryotic elongation factor 1α (eEF1A). Nuclear eEF1A was focalized in the nucleoli, and its expression was observed to increase following insulin treatment. Of the cPKC isoforms, only PKCβI was demonstrated to be expressed in the nucleus of C2C12 myocytes and to co-immunoprecipitate with eEF1A. In-depth analysis using site-directed mutagenesis revealed that PKCβI could phosphorylate Ser53of the eEF1A2 isoform and that the association between eEF1A2 and PKCβI was dependent on the phosphorylation status of eEF1A2.

Recent data indicate that some PKC isoforms are translocated to the nucleus, in response to certain stimuli, where they play an important role in nuclear signaling events. To identify novel interacting proteins of conventional PKC (cPKC) at the nuclear level during myogenesis and to find new PKC isozyme-specific phosphosubstrates, we performed a proteomics analysis of immunoprecipitated nuclear samples from mouse myoblast C2C12 cells following insulin administration. Using a phospho(Ser)-PKC substrate antibody, specific interacting proteins were identified by LC-MS/MS spectrometry. A total of 16 proteins with the exact and complete motif recognized by the phospho-cPKC substrate antibody were identified; among these, particular interest was given to eukaryotic elongation factor 1α (eEF1A). Nuclear eEF1A was focalized in the nucleoli, and its expression was observed to increase following insulin treatment. Of the cPKC isoforms, only PKCβI was demonstrated to be expressed in the nucleus of C2C12 myocytes and to co-immunoprecipitate with eEF1A. In-depth analysis using site-directed mutagenesis revealed that PKCβI could phosphorylate Ser53of the eEF1A2 isoform and that the association between eEF1A2 and PKCβI was dependent on the phosphorylation status of eEF1A2.

Interleukin 1 (IL‐1) delivers a stimulatory signal which increases the expression of a set of genes by modulating the transcription factor NF‐κB. The IL‐1 receptors are transmembrane glycoproteins which lack a catalytic domain. The C‐terminal portion of the type I IL‐1 receptor (IL‐1RI) is essential for IL‐1 signalling and for IL‐1 dependent activation of NF‐κB. This portion contains a putative phosphatidylinositol 3‐kinase (PI 3‐kinase) binding domain (Tyr‐E‐X‐Met), which is highly conserved between the human, mouse and chicken sequences, as well as the related cytoplasmic domain of the Drosophilareceptor Toll. This observation prompted us to investigate the role of PI 3‐kinase in IL‐1 signalling. Here we report evidence that PI 3‐kinase is recruited by the activated IL‐1RI, causing rapid and transient activation of PI 3‐kinase. We also show that the receptor is tyrosine phosphorylated in response to IL‐1. Expression of a receptor mutant lacking the putative binding site for p85 demonstrates that Tyr479in the receptor cytoplasmic domain is essential for PI 3‐kinase activation by IL‐1. Our results indicate that PI 3‐kinase is likely to be an important mediator of some IL‐1 effects, providing docking sites for additional signalling molecules.

Abstract:  The intracellular localization of phosphatidylinositol 3-kinase (PI 3-kinase) has been analyzed by western blotting, confocal, and electron microscopy immunocytochemistry in human osteosarcoma Saos-2 cells. By western blotting, the enzyme appears to be present in both the cytoplasmic and nuclear subfractions. By confocal microscope immunocytochemistry, the cytoplasmic fluorescence is localized in the perinuclear region and on a network of filaments, while a diffused signal is present in the nucleus, except for the nucleolar areas. Ultrastructural analyses on whole cells and on in situ matrix preparations reveal that nuclear PI 3-kinase is localized in interchromatin domains, in stable association with inner nuclear matrix components, while the enzyme diffused in the cytosol is partly associated with the cytoskeletal filaments. Quantitative evaluations indicate that, in a multidrug-resistant variant obtained by continuous exposure of Saos-2 cells to doxorubicin, the amount of nuclear and cytoplasmic PI 3-kinase is significantly lower than in the sensitive parental cell line. The nuclear localization of PI 3-kinase and its variation in multidrug-resistant cells, characterized by a reduced mitotic index, are consistent with the data on the existence of a nuclear inositol lipid cycle, which could also utilize 3-phosphorylated inositides to modulate signal transduction for the control of some key functional activities.

Tiazofurin treatment of K562 leukemia cells in vitrodepletes the metabolites of the guanylate biosynthetic pathway, inducing erythroid differentiation, that, in turn, alters the pheuotypic profile. As a consequence, K562 cells possibly modify their interaction with immune cells. Here we describe the binding and killing activity of peripheral blood NK cells against differentiating K562 cells and the correlation between their altered binding capacity and ICAM-1 expression levels in differentiating K562 cells. We found that decreased percentages of NK (and T) cells were bound to differentiating K562 cells generating a decreased cytotoxic activity. This corresponded to decreased expression of ICAM-1, as detected by FACS analysis and Western blot. Erythroid differentiation, binding and killing reduction, and ICAM-1 down-modulation were completely abrogated by guanosine treatment. Tiazofurin causes a decrease in lymphocyte recognition and binding to K562 target cells. This can be ascribed to the down-modulation of ICAM-1 expression on target cells, which, therefore, can escape killing, acquiring a selective survival advantage.

Monoclonal antibodies raised against DNA topoisomerase I and against topoisomerase II α and β isoforms, which have been previously demonstrated to be highly specific and capable of detecting cell cycle-related variations of the topoisomerase II isoforms (Negri et al.,1992, Exp. Cell Res.200, 452-459), have been utilized for a fine subcellular localization. Immunocytochemistry by confocal and electron microscopy have been used for a topological and quantitative evaluation of the fine distribution of the different topoisomerases in HeLa and K562 cells. Topoisomerase I and topoisomerase II α are present both in the nucleoplasm and in the nucleolus, though at different relative ratios, while topoisomerase II β is exclusively present at the nucleolar level. This is further confirmed by immunoblotting and immunocytochemical quantitative evaluations performed on purified nuclear matrix fractions obtained from K562 cells. In fact, the amount of topoisomerase I and topoisomerase II α present in the whole cell nuclei is partly lost in isolated nuclei but, while topoisomerase I is further significantly reduced in nuclear matrix preparations, the topoisomerase II α content is only slightly decreased. On the other hand, the great majority of topoisomerase II β is retained in the nuclear matrix and can be detected exclusively in association with the nucleolar remnant. These results are consistent with specific functional roles hypothesized for the different topiosomerase types.